Added coordinating information to offsetTree.Parentage.

Parentage objects now know the measure number, part name, start offset and stop
offset of their element.

They can also emit new parentages with different start and/or stop offsets via
a new() method. This will be useful once we implement mutators on the
OffsetTree, as we will be able to reposition elements within the tree while
maintaing their parentage information.